Immobilized inorganic diffusion barriers and the use thereof in the separation of small molecular species from a solution

ABSTRACT

The modification of a diffusion barrier by the deposition of one or more inorganic species within the pores and on to the surfaces of said diffusion barrier to form an immobilized crystalline lattice barrier or molecular grid having substantially uniform permeability and to select and separate different ionic species, giving a high efficiency of separation with a low energy input. The diffusion barrier is characterized by one or more units, each incorporating a tubular housing with a bundle of porous hollow fibres therein, arranged in the axial direction of the housing. The inorganic crystalline lattice is exemplified by barium sulphate (BaSO 4 ) formed by the interdiffusion of barium hydroxide (Ba(OH) 2 ) and sulphuric acid (H 2  SO 4 ) solutions, or, alternatively, co-precipitated aluminium hydroxide (Al(OH) 2 ) and barium sulphate (BaSO 4 ) formed by the interdiffusion of barium hydroxide (Ba(OH) 2 ) and aluminum sulphate (Al 2  (SO 4 ) 3 ) solutions. These immobilized inorganic diffusion barriers are used for selective separation of small molecules (e.g. with a molecular weight of less than 1000 daltons) and provide means for the more effective separation of ionic species. A method is also provided for the low cost removal of ions from molasses, to enhance fermentation efficiency and to improve its usefulness as a stock feed supplement, or it may be followed by other treatments to reduce and/or remove higher molecular weight substances, such as proteins and waxes, to produce sugar syrups and caramel substitutes.

CROSS REFERENCE TO RELATED APPLICATION(S)

This United States application stems from PCT International ApplicationNo. PCT/AU/83/00023 filed Feb. 2, 1983.

TECHNICAL FIELD

The present invention relates to the modification of diffusion barriersby the immobilization of inorganic species therein to increase theselectivity of the barrier, and to the use of such barriers in theseparation of small molecularspecies from a solution.

BACKGROUND ART

Various types of membrane diffusion barriers are known according to thepublished prior art. Molecular and ionic species diffuse through suchmembranes according to the value of their diffusion coefficient, and forthat reason their selectivity is generally poor. Also known according tothe prior art are liquid membranes, immobilized liquid membranes orinterfacial polymerization membranes which, whilst useful in certainspecific applications, suffer the same defects as other previously knownmembranes.

Small molecular weight separations are not easily achieved by economicalindustrial stage processes. Classical hyperfiltration requires highpressure and high-equipment cost for the low selectivity, low fluxes andthe always present risk of membrane fouling. Chromatography, includinghigh pressure liquid chromatography, gives excellent selectivity but islimited to laboratory scale separations, and is a too costly operationfor most industrial applications. Ion exchange, ionic exclusion,transport depletion and reverse electrodialysis are processes applicableonly when macro-molecules, with isoelectric points different to theoverall pH, are not present in large quantities, otherwise poisoningoccurs. Furthermore, high investment cost and high operating costprohibits their use for the treatment of cheap feedstocks for thefermentation industry.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a diffusion barrier for afluid medium (gas or liquid) with a porosity greater than about 10-20%for channel dimensions of the order of only several Angstrom.

It is also an object of this invention to provide a selective diffusionbarrier wherein a predetermined inorganic species is immobilized withinthe matrix thereof, to produce a diffusion barrier having asubstantially uniform molecular grid structure which functions as aneffective molecular screen for the selective separation of smallmolecules (e.g. molecular weight less than about 1,000 daltons) andprovides means for the more effective separation of ionic species. Thematrix used can be any known diffusion barrier e.g., sintered glass,dialysis membranes, porous metals, or any other suitable porousstructure.

It is another object of the present invention to utilize existingdiffusion barriers as a support to undergo chemical reaction tomanufacture a substantially uniform grid of precise molecular size, withregard to the molecular size of the species to be separated.

It is an object of the present invention to provide a substitute for ionexchange; to provide a method of separating ionic species andnon-charged species from an organic media wherein said method has thesame effect as direct ion exchange treatment but which avoids thedisadvantage of poisoning of the ion exchange resin.

It is a further object of this invention to provide a method of, andmeans for, the separation of small ionic or molecular species from aliquid and for fractionation of said liquid by the use of a diffusionbarrier having a uniform pore or molecular grid structure or uniformpermeability, wherein the diffusion of small ionic or molecular speciesis obtained against the osmotic current through said molecular gridstructure to achieve a selectivity greater than the ratio of therelative diffusion coefficient of the separated species. As used herein,the phrase "against the osmotic current" is meant to signify the factthat in the presently claimed processes small molecular species(solutes) diffuse through the counter diffusion barrier from the sidehigher in solute concentration to the side lower in soluteconcentration, whereas in a conventional osmotic process a solvent orliquid will pass through the diffusion barrier from the side lower insolute concentration to the side higher in solute concentration.

It is another object of this invention to provide a method of usingunitary separative units to increase the original separation factor ofsuch units by use of a cascade system; to provide a method of increasingthe separation factor between two components in a liquid phase which arerequired to be separated, without using the multi-stage recycling ofclassical cascade systems.

It is yet a further object of this invention to provide a method for thedesalting of molasses to upgrade the value thereof and to provide thebasis for improved sugar recovery therefrom or for the production of aliquid sugar product or a product equivalent to high test molasses. Itis also an object to provide an improved feedstock for fermentationwhich increases the rate of fermentation, the yield of alcohol or otherproducts and the solids concentration of the effluent.

These and other objects of the invention will be further discussed inthe following description of the invention.

According to one aspect of the present invention there is provided amethod of immobilization of an inorganic material inside a diffusionbarrier in the form of a colloidal-, crystalline-, semicrystalline-species, or a mixture thereof, with control of the physical structure ofthe immobilized species, and without plugging of the outside surfaces ofthe diffusion barrier. A diffusion barrier so treated, in most caseswould not normally lose any more than about 10% of its originalpermeability.

According to another aspect of the invention there is provided a processfor the separation of small molecular species from a primary liquid(e.g. molasses or molasses fermentation dunder) containing one or aplurality of dissolved molecular species of different molecular weight,comprising subjecting said primary liquid to treatment with a diffusionbarrier having substantially uniform matrix permeability which separatesthe primary liquid from a solvent (e.g. water), wherein an osmoticgradient exists between the primary liquid and the solvent, and wherebysmall molecular species, especially salts, are selectively transferredfrom the primary liquid to the solvent against the osmotic current.

The diffusion of the small molecular species against the osmotic currentprovides a selectivity greater than the ratio of the relative diffusioncoefficient of the separated species.

Although the diffusion barrier to be used may be any suitable porousdiffusion medium having uniform permeability to enhance the selectivityof the separative process, the preferred diffusion barrier compriseshollow fibres contained in a hollow fibre tube unit having substantiallyuniform permeability, wherein the primary liquid is circulated throughthe inner channels or lumens of a bundle of hollow fibres and whereinthe solvent is circulated in the space surrounding the bundle of hollowfibres, whereby the small molecular species can diffuse through themolecular screen provided by the uniform grid structure in the walls ofthe hollow fibres against the osmotic current and into the solvent.

In another aspect of the present invention the solvent circulating inthe space surrounding the bundle of fibres is further treated in acascade system to separate the molecular species contained therein.Preferably the solvent circulating in the space surrounding the bundleof hollow fibres is circulated to the space surrounding the bundle ofhollow fibres of a second hollow fibre tube unit wherein an osmoticgradient exists between the solvent in said space and a further solventcirculating through the interior channels or lumens of the hollow fibresof the second hollow fibre tube unit, whereby small molecular speciesare selectively transferred from the solvent in said space to thefurther solvent circulating through the interior channels or lumens ofthe hollow fibres of the second hollow fibre tube unit. The treatedsolvent from the space surrounding the bundle of hollow fibres of thesecond hollow fibre tube unit is normally circulated back to the spacesurrounding the bundle of hollow fibres of the first hollow fibre tubeunit. In order to regulate or to control the concentration of dissolvedmaterial being recirculated with the solvent in the coupled hollow fibretube units, a part of the treated solvent from the space surrounding thebundle of hollow fibres of the second hollow fibre tube unit iscirculated back to the stream of primary liquid to be treated by thefirst hollow fibre tube unit. Pure solvent (e.g. water) can be added tothe circulating solvent stream to replace the volume of solvent whichhas been circulated back to the stream of primary liquid (e.g. molasses)to be treated.

It is also within the scope of the invention for the solvent to befurther treated by ion exchange or other treatment capable of fixing byadsorption the species to be removed from the solvent.

According to a further aspect of the present invention there is provideda process for the fractionation of a primary liquid (e.g. asugar-containing solution such as molasses, or dunder from thefermentation of molasses) containing a plurality of dissolved molecularspecies of different molecular weight, comprising:

(i) subjecting the primary liquid to treatment with a diffusion barrierwhich separates the primary liquid from a second liquid (e.g. water),wherein an osmotic gradient exists between said primary and secondliquids and whereby small molecular species (.e.g. salts) areselectively transferred from the primary liquid to the second liquidagainst the osmotic current; and

(ii) subsequently subjecting said primary liquid to ultrafiltration witha semipermeable membrane,

permitting the passage therethrough of water or other solvent componentof said primary liquid together with molecules below a predeterminedsize contained therein (e.g. sugar molecules), to form a permeate oftreated product containing said molecules, and

preventing the passage of molecules above said predetermined size (i.e.macromolecular species, such as proteins) to form a concentrate orretentate of the remaining components of the primary liquid.

The invention also relates to products--such as molasses or dunder fromthe fermentation of molasses--which have been treated by theabove-described processes and to the fractionated products--such assugars (in liquid or crystalline form) and protein--obtained as a resultof such processes.

In yet another aspect of the present invention there is providedapparatus for the fractionation of a primary liquid containing aplurality, of dissolved molecular species of different molecular weight,comprising:

(i) porous diffusion barrier means with a molecular grid matrix andhaving substantially uniform grid permeability to enhance selectivityand being adapted to separate said primary liquid from a second liquidwhereby small molecular species may diffuse from said primary liquid tosaid second liquid under the influence of an osmotic gradient betweenthe primary and second liquids; and

(ii) ultrafiltration means whereby said primary liquid may subsequentlybe selectively split into a permeate of a product containing solvent andmolecules below a predetermined size, and a concentrate or retentatecomprising molecules above said predetermined size.

Ideally, the uniform grid permeability of the diffusion barrier is suchthat the diffusion of the small molecular species against the osmoticcurrent results in a selectivity greater than the ratio of the relativediffusion coefficient of the separated species.

Preferably, the diffusion barrier means comprises one or more hollowfibre tube units, each comprising a tubular housing; a bundle of poroushollow fibres arranged within the housing in the axial directionthereof; a countercurrent liquid chamber formed between the outersurfaces of the hollow fibre bundle and the inner surface of thehousing; first inlet and outlet ports for passing the second liquid intoand out of the liquid chamber; partition walls supporting the hollowfibre bundle, separating the open ends of the hollow fibres from theliquid chamber and defining the length of the liquid chamber; and secondinlet and outlet ports for the primary liquid phase, the second inletand outlet ports communicating with the interior space or lumen of eachof the hollow fibres, and wherein a plurality of channels ofpredetermined molecular dimensions and permeability communicate betweenthe interior space or lumen of each hollow fibre and the liquid chamber.

Preferably, in order to provide a system whereby the second liquidcirculating in the countercurrent liquid chamber may be treated toremove dissolved ionic or molecular species therefrom, for example asmay be required for environmental considerations, a cascade system maybe provided wherein the first inlet and outlet ports of each hollowfibre tube unit are connected in series to the first inlet and outletports of a second hollow fibre tube unit for circulation of the secondliquid therebetween.

In a further modification thereof, an open cascade system is providedwherein means are provided for directing at least part of the treatedsecond liquid back to the stream of primary liquid to be treated by saidapparatus, and wherein further means are provided to add pure secondliquid (e.g. ultrafiltered water) to replace the treated second liquidwhich had been removed.

The cascade system contains a solvent for the stripped molecule, whichcan be different to the solvent used for the primary liquid, and for theoutside stripping current of the second unit, provided that if thissolvent is different, it is not miscible with the other two solvents.

As a further modification of the invention, ion exchange means and/orother means capable of fixing the dissolved species by adsorption may beprovided to remove compounds from the second liquid which have diffusedtherein from the primary liquid which have diffused therein from theprimary liquid. For example, the apparatus may also comprise one or moreof the following features.

(i) one or more ion exchange resins to selectively remove the ions orcharged molecules required to be stripped, and which can be regeneratedby classical ion exchange using a strong base or a strong acid.

(ii) one or more Sirotherm-type units, as developed by the CSIRO, whichcan strip the current from charged species and which can be regeneratedby thermic treatment.

(iii) one or more adsoprtion units e.g., activated carbon, which can beregenerated by thermic treatment.

(iv) one or more metastable ionic systems which can be regenerated bydifferential pressure.

(v) a solvent inlet, if the solvent used is the same as the base solventof the primary liquid being treated, to maintain the volume of theclosed circuit system constant, and to equilibrate the transfer ofsolvent due to the difference in osmotic pressure between the inner coreand the outer core of the first unit, and in this particular case takinginto account the lesser transfer of the same nature occurring in thesecond unit.

(vi) a unit for selective separation of a two-phase liquid when theclosed circuit uses a two-component fluid of non-miscible solvent--as insolvent extraction where one solvent is the solvent used in the primaryliquid being treated, whereas the other solvent is a better solvent forthe species to be stripped than is the first mentioned solvent.

(vii) a device to make an emulsion of one solvent in another whenfeature (vi), above, is used.

(viii) a pump to ensure recirculation.

(ix) an outlet from the closed circuit to bleed the product inrecirculation back to the original product to be treated.

Classical dialysis processes use membranes, isotropic or anistropic,with pore dimensions small enough to limit water convection by asufficient amount so as to avoid interference between molecules andtheir first sphere of hydration, and the wall. Under thesecircumstances, classicial diffusion laws apply, and dialysis isperformed in direct relationship with the respective values of themolecular speices concerned. Pore uniformity, in this case is notcrictical and the value of measured diffusion coefficients are inessence the same as the ones measured in sintered glass diffusion cells(this being corrected using the relative value of membrane resistanceand interface resistance (see Scheitzer, "Handbook of SeparationTechniques", McGrawhil).

However, with a uniform grid of molecular dimensions close to half themean free path of the respective molecules, selective counter dialysisoccurs. This effect is possible only if none of the pores offer freepassage to solvent detrimental to the equilibrium water flux/diffusion,or, in other words, if bulk back diffusion is minimal.

In this process, a membrane or other diffusion barrier is interfacedwith two fluids: one with high osmotic pressure, the other with lowosmotic pressure. Fluid 1 contains various low molecular weight species,to be selectively transferred to Fluid 2. If we consider two molecularweight species (a) and (b) with bulk concentration in Fluid 1 beingC_(1A) and C_(1B) transferable through the membrane pores, each speciestransfers according to their relative concentration gradient and theirdiffusion coefficient with local transport vectors D_(A) and D_(B). Acounter current of water inside the pore, corresponds to a localtransport vector D_(W). D_(A) and D_(B) are directly related to the thinwall concentration gradient (i.e. C'_(1A), C'_(1B), C'_(2A), C'_(2B)replaced C_(1A), C_(1B), C_(2A), C_(2B) in respective equations). In thesame manner D_(W) depends on the real osmotic pressure differencebetween the two membrane walls and not the osmotic pressure differencemeasured from the bulk concentration of the species (due toconcentration polarisation). Variation of C'_(1A) versus C_(1A), or moregenerally C' value versus C values, are governed by the Blatt equationwhich expresses the influence of shear rate on concentrationpolarisation. For a given value of C_(1A) it is always possible tochoose a value of D_(W) such as

    |D.sub.B |<|D.sub.W |<|D.sub.A |

by adjusting the shear rate values relative to Fluid 1 and Fluid 2 orthe pressure gradient across the membrane to selectively modify D_(W).

However, in practice, the dimensions of the modified pore structure(i.e. from about 50 Angstrom to about 10 Angstrom) are such thatphenomena other than classical diffusion have to be taken into account,and the present invention utilises the maximum interaction between theimmobilized inorganic species, the ionic species in solution and/or thefirst or second sphere of hydration.

In this regard, at least four separate phenomena have to be consideredas complimentary to classical diffusion:

(i) Anomalous diffusion phenomena. In the example given above D_(A) andD_(B) can be influenced by the relative local concentration gradients ofthe two species, the less mobile ion being slowed down by the mostmobile ion--e.g., K⁺ diffuses faster than Na⁺ ; if the concentration ofNa⁺ increases then the rate of diffusion of K⁺ will increase whilst thatof Na⁺ decreases.

(ii) Donnan equilibrium effect to respective electroneutrality oftransfer.

(iii) Ionic species which are associated with water through strong ortight bonds for their first sphere of hydration, are generallyassociated with a second sphere of hydration through loose or weakbonds, which can be explained only by the partial orientation of themolecule of water or solvent outside of the first sphere or hydration.In this case the second sphere of hydration is defined by a level ofenergy being the difference between the energy state of the randommolecule concerned, and the partly re-entered state. The channeldimensions at the level of the second sphere of hydration are used toselectively influence the diffusion of the species. In this regard, theminute difference between the energy levels involved in the coordinationof the solvent molecules inside the second sphere of solvation allowsfor discrimination between ionic species which have the same outer shellelectron structure (e.g. between hafnium and Zirconium).

(iii) Close molecular interactions between wall, solute and solvent. Thecoordination energy between a wall and an adjacent ionic species can beused to differentiate between ionic species, and this energy is linkedto the solubility product of the molecular species obtained by thecombination of oppositely charged ions in the wall. For example, ifBarium Sulfate is used as the major component of the grid, SO₄ ²⁻ ionswill not diffuse, or will diffuse much slower, because the solubilityproduct [Ba²⁺ ] [SO₄ ²⁻ ] is extremely small, but Cl⁻ ions will diffusewithout measurable difference compared to the normal diffusioncoefficient, because the solubility product [Ba²⁺ ] [Cl⁻ ]² is large.

BEST MODE OF CARRYING OUT THE INVENTION

Expressed in another way, a reaction is effected between two reactants,each in the form of a solvent and a solute circulating on opposite sidesof a primary matrix. For example, if the matrix is in the form of ahollow fibre th reaction is conducted between two solutions `A` and `B`,containing solutes `a` and `b`, respectively, circulating on oppositesides of the hollow fibre matrix, and `A` on the outside and `B` on theinside.

The components to be immobilized are insoluble in both solvents a' andb', or at least totally insoluble in one of them.

For immobilization of a predetermined compound `c`, obtained by thereaction between solutes `a` and `b` according to the following reactionscheme:

    a+b→c↓+d

the active ionic parts of `a` and `b` each have their stoichiometricproportions determined by the reaction, but corrected by their diffusioncoefficient or their ionic mobility.

For example, the immobilization of Barium Sulfate (BaSO₄) inside ahollow fibre-type membrane is effected by the reaction between BariumHydroxide (Ba(OH)₂) solution outside the hollow-fibre and Sulfuric Acidsolution (H₂ SO₄) inside the fibre. The rate of reaction is controlledso as to have stoichiometric proportions of Ba²⁺ and SO₄ ²⁻ inside themembrane at the start of the reaction.

In other words the value of the diffusion coefficient for SO₄ ²⁻ is usedto correct the concentration of H₂ SO₄ used to have the same ionicconcentration as Ba²⁺ at the interface. Expressed in another way, it isnot sufficient to simply determine the stoichiometric proportions of thetwo solutions which are used, as it is also necessary to take intoaccount the different speeds of diffusion. Diffusion coeffieients ofvarious ionic species can be found by reference to standard texts.Multiplication of the appropriate diffusion coefficient by theconcentration of the ionic species gives the quantity of ions which willbe present at a certain time at the interface. With this figuredetermined it is then necessary to check the quantity of ions of theother species which will be present and to adjust the concentration ifnecessary such that exactly the same concentration or stoichiometricproportion will be present at the interface, concurrently. In respect ofthe formation of crystals of inorganic species in the membrane,allowance can be made as far as the diffusion coefficients are concernedby the variation of such diffusion coefficients versus temperature ifthe temperature of the two streams are not the same.

A slight disequilibrium, with a higher concentration of component `a`,will create immobilization on the side in contact with component `b`. Inother words, in the example given above, if the concentration of Ba(OH)₂which is used is higher than the predetermined concentration, theBa(OH)₂ outside of the hollow fibre will cause the immobilization ofBaSO₄ on the inner core of the hollow fibre.

In the general reaction sequence given above, any reaction between `a`and `b` which produces at least one insoluble product in one of thesolvents contained by solutions `A` and `B` can be used.

The density of the precipitate will be highest in the layer which isformed initially, but will be gradually lower for subsequently formedlayers inside the fibre, extending to the outside side of the hollowfibre.

It is also within the scope of the present invention to immoblize twodifferent layers of inorganic species in and on the diffusion barrier,one inorganice species giving selectivity for one specific component,the other inorganic species given selectivity for one or more othercomponents contained in the solution being treated.

In practice it is normally possible to take any inorganic compound whichis not soluble in one of the solvents which is being used. From thechemical formula of this compound it is possible to determine thereaction scheme of sequence which will lead to that compound. With twocomponents in the reaction, the diffusion coefficients are determinedfor each component, and based on the teachings of the present inventionit is possible to determined the requirements to effect immobilizationof the compound.

The preferred diffusion barrier is a porous hollow fibre wih a wallthickness of about 3 to about 15 microns and an internal channel heightor diameter of about 200 microns.

In an embodiment of the invention relating to the desalting of molasses,the diffusion barrier used was a 1.8 square metre hollow fibre unit,treated such that Al(OH)₃ and BaSO₄ were immobilized therein as aco-precipitate, obtained by the reaction between Al₂ (SO₄)₃ and Ba(OH)₂.To work out the precise concentrations required the above describedtechnique (determining diffusion coefficients, etc) is followed, withthe regeneration of a solution of saturated Ba(OG)₂ outside the hollowfibre and 0.158N H₂ SO₄ inside. With this system, typically 500 gm ofmolasses is diluted with 500 ml of H₂ O, giving a total volume of 830ml, and this solution is circulated inside the hollow fibre with a flowrate of 400 ml/minute. Outside the hollow fibre H₂ O is circulated at aflow rate of 2 liters/minute.

With such diffusion barriers of the type referred to above, themechanism for selectivity is of five types:

(1) Selectivity by ionic interaction. When you have an inorganicprecipitate as a barrier, the remaining charge influences the flow ofmolecules by proximity influence, e.g., BaSO₄ is selective against SO₄ ⁻but allows Cl⁻ to pass.

(2) Differential site-to-site transport, e.g., an hydrophobic barrierwill selectively transport any agent which can be solvated, especiallyany plasticizer of such a barrier against water. Site-to-site transportis a step-by-step operation, which includes:

(i) adsorption of molecules;

(ii) diffusion through the barrier, and

(iii) desorption from the barrier to the second layer.

(3) Selectivity by virtue of the osmotic gradient between two liquidphases separated by the barrier. In this case, solvent counter-currentopposes the transport of the diffusing species. If the maximum griddimension is less than the free path of the diffusing molecule, thecounter-current effect of solvent makes a selective separation againstthe apparent surface of resistance offered by the molecule to the flowof solvent transported back.

(4) Anomalous diffusion, as described above.

(5) Selective counter dialysis, as described above.

Unfortunately, these selective modes--and especially those outlined in(2) and (3), above--are not 100% effective. In the case where it isrequired to make a separation between small ionic species and smallnon-charged species, it is difficult to obtain a completeseparation--and especially to completely block the transport ofnon-charged species e.g., in the case of the desalting of, or theselective removal of ionic species from, a complex organic media, whereit is required to avoid the transport of small organic molecules, suchas in the desalting of any sugar syrup (e.g. molasses).

One classical prior art method to achieve such a separation is to useion exchange resins, which give in two steps the deionization of thecomplex mixture.

Unfortunately, ion exchange sites prepared in a microporous matrix arevery sensitive to poisoning by large organic molecules which are, inmost cases, present in the original media. For this reason it isimportant to avoid any contact between molecules bigger than the innerpore structure of the ion exchange resin, and the resin itself.

The invention will be further described with reference to an embodimentthereof relating to the desalting of cane sugar molasses.

In general terms, molasses contains about 55% sugars by weight (80% byvolume), 25% water and approximately equal amounts of salts and largeorganic molecules. The large molecules (molecular weight above 20,000)are mainly proteins and waxes and the salts primarily comprise K, Mg, Naand Ca ions associated with chlorides and sulphates as counter ions.

The sucrose content in molasses varies between 25 to 40%, and thereducing sugars range from 12 to 35%, but the sum of the two generallyconstitutes at least 50 to 55% of the final molasses.

However, the accumulation of salts and higher organic molecules inmolasses constrains economic sugar recovery therefrom, and reduces itsvalue in fermentation and in the stock food industries.

The high salt content of molasses, especially potassium salts inmolasses from sugarcane, has limited its final utilization. Thecombination of high ash and non-sugars content in molasses limit therecovery of residual sucrose due to complex solubility reactions. Inaddition the high potash levels in molasses have a laxative effect whichlimits molasses utilization in animal feeds.

Currently available techniques for salt removal from molasses are basedon ion exchange and electrolysis. However, because the high molecularweight components in molasses cause resin and membrane fouling nocommercially viable process has been developed for the cane sugarindustry.

By means of the use of the present invention it is possible to removethe bulk of the salts (potassium, calcium and magnesium salts inparticular) and other small molecules from the molasses to providetreated molasses which is a more useful product for use in moreefficient fermentation and for stock food applications.

By subjecting the treated molasses to specific ultrafiltrationtreatment, it is possible to effectively remove the macromolecularorganic compounds (wax, proteins, polycellulosic material) thus creatingthe conditions for the complete recovery of the sugar content of themolasses, e.g. in the form of a sugar syrup, from which the sucrose canbe crystallised, if required. By a combination of hollow-fibreseparation and ultrafiltration technology it is possible to deplete theinput molasses of a major portion of its salt and large MW species. Thisyields a sugar syrup of a product equivalent ot treacle or "goldensyrup".

According to this embodiment of the invention, the molasses is subjectedto an osmotic desalting treatment in a series of hollow-fibre separationunits. Preferably, each separation unit comprises a highly selective andhighly porous diffusion barrier, with the hollow fibres consisting ofregenerated cellulose, being of substantially uniform pore structure andwherein a predetermined inorganic species in the form of a crystallinelattice is immobilized within the hollow-fibre structure, both in themembrane pores and on the surfaces. In this way each hollow-fibrecomprises a substantially uniform grid of precise molecular size, withregard to the molecular size of the species to be selected or separated.The type of crystalline lattice can be changed to vary the rejectioncharacteristics of the diffusion barrier.

Molasses is passed through the hollow fibres countercurrent to a streamof water (or ultrafiltered water) flowing around the outside of thehollow fibres. Each hollow-fibre unit has an inlet (with pre-filter) andan outlet for the molasses feed and an inlet and an outlet for thecounter-current water flow. The pores are such that they will preventtransfer of sugars to the countercurrent water flow, which is furtherinhibited by the inward flow of water molecules. However, potassium,calcium and magnesium ions are able to move counter to this flow via anionic diffusion transport mechanism, as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment of the invention relating to the desalting of molasseswill be further described with reference to the drawings, in which:

FIG. 1 illustrates a schematic representation of apparatus embodyingprinciples of the present invention and comprising one or more hollowfibre desalting tube units, possibly in one or several banks of suchtubes connected in series or in cascade form, depending upon the levelof desalting required, and further comprising one or more banks ofultrafiltration units to separate the sugars from the macromolecularcomponents of the molasses, such as proteins and waxes.

FIG. 2 illustrates an open cascade arrangement for the desalting ofmolasses designed for the recovery of the bulk of any of the sugarcontent of the molasses which may have bled into the countercurrentwater and which is required to be treated before drainage thereof.

Each hollow fibre desalting unit according to the preferred embodimenthas an active surface area of 1.8 m² and comprises a plurality (9,000 to15,000) hollow fibres with a wall thickness of about 3-15 microns, aninternal channel or lumen diameter of about 200 microns. The hollowfibres have a porosity ratio of about 20% with pores or molecularscreens of about 10 Angstrom.

As shown in FIG. 1, molasses--diluted with water if necessary--is pumpedthrough the internal channels of the hollow fibres with filtered orultrafiltered tap water pumped in countercurrent in the spacesurrounding the hollow fibres. Under the influence of the osmoticgradient which exists, certain ions--including potassium, calcium andmagnesium ions--diffuse through the molecular screen provided by theporous wall structure of the hollow fibres, with some water flowing backthrough the channels and into the molasses stream. According to the flowrate of the molasses pumped through the hollow fibre tubes, and thelevel of desalting which is required, the molasses may be passed throughone or a plurality of banks of such hollow fibre tubes (with tubes ineach bank in parallel) to obtain the required level of desalting. Forexample, the potassium content may be reduced by up to about 90% of theoriginal potassium content, depending upon the number of hollow fibretube units used and the particular operating conditions applied. Morethan half of one of the most prolific salts in molasses, potassium, canbe removed in a single pass through a bank of hollow fibre tubes. A 60tube module can produce 1.8 tonnes per hour of substantially desaltedmolasses while using less than 4 m³ of water per hour with a powerconsumption of less than 4 KW. Operating pressures are typically lessthan 50 kpa but depend on the viscosity of the molasses, temperature,flow rate and the desired level of desalting.

Ion levels in the molasses can be determined by atomic absorptionspectroscopy before and after passage through the hollow fibre tubes.

The following results have been obtained for a 1.2 m² membrane usingmolasses diluted to 520 g/l sugar concentration:

    ______________________________________                                                 Ion Concentrations (g/l)                                             No. of Passes                                                                            K.sup.+  Na.sup.+                                                                             Mg.sup.+                                                                             Ca.sup.++                                                                           Cl.sup.-                              ______________________________________                                        Control    41.4     1.1    4.6    8.4   21.3                                  1 Pass     26.0     0.72   3.2    7.0   12.4                                  2 Pass     20.0     0.60   2.5    5.0    9.5                                  3 Pass     14.0     0.49   2.0    4.6    6.8                                  ______________________________________                                    

At this stage, if required, the treatment of molasses can be terminatedand the desalted molasses returned to the refinery or to thefermentation plant.

However, the desalted molasses may be further treated, in a secondphase, by subjecting the desalted molasses to ultrafiltration, toeliminate macro-molecules which have a molecular weight higher than thesugar, i.e. to remove the protein and the wax.

By a combination of hollow-fibre separation and ultrafiltrationtechnology, as shown in FIG. 1, it is possible to deplete the inputmolasses of a major portion of its salt and large MW species. Thisyields a sugar syrup or a product equivalent to "Golden Syrup".

The desalted molasses is circulated through a three-stageultrafiltration system of capillary modules, as shown in FIG. 1.Ultrafiltration effects the separation of different molecular sizesthrough porous membranes in thin channels at high velocity. Variousmolecular weight cut-offs are available to adjust the wax andmacromolecular content of the molasses to meet market needs andrequirements.

The combined effect of the hollow fibre desalting tube and theultrafiltration equipment permits the removal of the small molecularfraction and the high molecular fraction thus leaving a syrup whichcontains the sugars (sucrose and invert sugars) and some colouringagents of intermediate molecular weight. This product (treacle of goldensyrup) can be either:

Directly used as a liquid sugar substitute and eventually a naturalcolouring agent (the colour can be adjusted by modification of themolecular weight cut-off of the UF membrane).

Concentrated by multiple effect evaporation to be shipped and used insensitive fermentation areas (baker's yeast, citric acid, mono sodiumglutamate, pharmaceuticals).

Returned to the refinery for sucrose extraction; the low potassiumcontent and the absence of high molecular compounds permits two thirdssucrose recovery per strike.

The equipment is readily tuneable or adaptable to the user'srequirements; it is a simple matter to adjust the apparatus to modifythe salt content of the product.

Furthermore, the use of different ultrafiltration membranes allows themanufacturer to selectively control the amount of small moleculesrelated to color, taste and surfactant properties for enhancement ofcolour, taste and eventually mechanical properties of the end product.

The first two stages of the ultrafiltration unit produce the sugarsyrup. In the third stage water is added to rinse the molasses. Thepermeate from Stage III (very dilute syrup "sweet water") is recycledback to the counter-current mixer where it is used to dilute theincoming molasses prior to the desalting stage. Because it is a closedloop system, there is no loss of sugar.

The depleted molasses flow from Stage III is primarily a proteinconcentrate, containing about 25% protein and wax W/W. For smallinstallations and protein concentrate will simply be discharged to ananaerobic septic tank. For larger installations (e.g. treating in excessof 10,000 tonnes molasses/year) it may be feasible to install a dryer torecover the protein concentrate in powder form. The protein concentratesolution is stable only for about 3 days but when dried it has a stableshelf life of about 12 months.

FIG. 2 of the drawings illustrates an open cascade modification for thehollow fibre tube desalting unit, whether for use as part of the systemillustrated in FIG. 1, or for a straight molasses desalting applicationwithout the subsequent ultrafiltration treatment.

As shown in FIG. 2, molasses is pumped through the central channels ofthe hollow fibres of Tube 1 with water pumped at countercurrent in thespace surrounding the hollow fibre bundle. To reduce the effect of anysugar bleeding from the molasses into the countercurrent water, saidcountercurrent water is pumped into the space surrounding the hollowfibre bundle of a second tube (Tube 2) which has been connected toTube 1. In Tube 2, filtered water is pumped through the central channelsof the hollow fibres wherein, under the influence of an osmoticgradient, salts in the water surrounding the hollow fibres diffusethrough the porous walls of the hollow fibres and into the water whichis circulating through the central channels of the hollow fibres, afterwhich it is passed to the drainage system. The bulk of the sugar whichhave bled into the water surrounding the hollow fibre bundle of Tube 1are normally retained in the water circuit between Tubes 1 and 2.

In order to control the level of sugars circulating in the water betweenTubes 1 and 2, part of this water is bled off (as shown in FIG. 2) andis recirculated back to the incoming molasses stream where it serves auseful purpose of diluting the feed molasses to an acceptable level ofdilution for pumping through the thin channels of the hollow fibres. Thesugar-containing water which is bled off is replaced by ultrafilteredwater which is feed into the looped circuit between Tubes 1 and 2, asshown in FIG. 2.

In one experimental application molasses containing 50% by weight ofsugars and 5% salts was passed through Tube 1. The resulting desaltedmolasses from Tube 1 contained 49% sugars and 2% salts (i.e. 60%desalting). Consequently, the countercurrent water in Tube 1 contained3% salts which had diffused through the walls of the hollow fibres fromthe molasses into the countercurrent water, as well as the 1% of sugarswhich had bled into the water.

When the countercurrent water was passed into the space surrounding thehollow fibres of Tube 2, and was treated against pure water circulatingthrough the central channels of the hollow fibres of Tube 2, testsindicated that whereas 1.8% of the salts passed into the central channelwater, only 0.02% of the sugars bled into this water which was to bedirected to drainage.

Thus the water to be recirculated back to Tube 1, or which was to berecirculated back to the incoming stream of molasses, contained thebalance of 0.98% of the sugars and 1.2% of the salts. Thus by the opencascade system illustrated in FIG. 2, any loss of sugars by bleeding canbe reduced to a minimum, and to an environmentally acceptable level.

The form of the apparatus to be used for the treatment of molassesdepends upon the intended end use(s) of the treated molasses and theenvironmental considerations which must be taken into account at theintended site of operation. Generally, three basic modes of operationare provided.

TYPE 1: Where pollution control is not of primary importance and whenthe investment has to be minimised, water (preferably ultrafiltered) isfed directly to the outskirt of the tube and molasses to the innerchannel (1 kg/mn/tube of molasses for 1.51/mn per tube of water). One,two or three passes (one, two or three tubes in series ofcounter-current) can be used to achieve the necessary level ofdesalting.

TYPE 2: Where pollution control is important (when the factory is notallowed to reject more than 600 ppm of BOD) a double-tube, open-cascadearrangement, as shown in FIG. 2, is used. This equipment is well adaptedto the fermentation industry where a high level of dilution is required.

In this case (Cascade Desalting) the use of UF water is limited to thelevel of dilution needed for fermentation and the desalting water issimply filtered.

TYPE 3: Where pollution control is paramount (when the factory is notallowed any reject in the form of BOD) water consumption has to beminimal and the dilution of the molasses has to be minimised (forexample, for further crystallisation of sugar) an ion exchange resinunit replaces tube number two in the cascade arrangement illustrated inFIG. 2.

For each type the following parameters can be defined:

Desalting level expressed in potassium reduction in % (K%)

Sugar Loss expressed in (invert Equivalent) % (S%)

Separation factor=K%/S%

    ______________________________________                                                    TYPE 1   TYPE 2   TYPE 3                                          ______________________________________                                        Maximum Desalting                                                                           95%        50%      99%                                         Separation Factor                                                                           2 steps    (30-80)  1 step                                                     (6-10)             (complete                                                                     separation)                                 Sugar loss per                                                                              3%-5%      0.2%-1%  0                                           step (as invert)                                                              UF water per kg                                                                             1.5         0.3-3*  0.3**                                       of molasses                                                                   Filtered water                                                                              --         1.5                                                  ______________________________________                                         *according to dilution                                                        **volume in liters                                                            Potassium  reduced 2-3 fold                                                   Sodium  reduced 2-3 fold                                                      Calcium  reduced 4 fold                                                       Magnesium  reduced 2-3 fold                                                   Iron  not affected                                                            Copper  not affected                                                          Molybdenum  not affected                                                 

In the case of a manufacturer fermenting molasses to produce alcohol,the step of desalting molasses makes his fermentation more efficient inthe following ways:

(i) Increased reactivity:

Lower potassium and chlorine levels, results in shorter fermentationtimes. As this is the most time-consuming operation, and the steprequiring the largest capital invention, the manufacturer now has thecapability to treat more molasses in any given period. For example,using Zymomonas mobilis as the micro-organism for fermentation of a 9%sugar concentration, the following fermentation times were obtained forthe same level of sugar transformation:

(a) untreated--34 hours

(b) 1 pass desalting--9.5 hours

Similarly, using the yeast Saccharomyces uvarum as the micro-organism,fermentation time was reduced from 32 hours (for untreated molasses) toabout 16 hours (using desalted molasses). The actual increase infermentation capacity a fermenter can obtain by using the desaltingprocess will depend on such factors as the type of molasses used, thesugar concentration and the type of fermentation micro-organism used.

(ii) Increased Yield:

It is possible to transform more sugar to alcohol from each batch ofmolasses. This means that a manufacturer can produce more alcohol from agiven quantity of molasses. Increased Alcohol Level in the `Beer`:

A most important advantage is that at equilibrium (or even before) thelevel of alcohol that can be produced in the substrate is higher thanthe amount of alcohol that can be produced in non-treated molasses. Thisgiven three advantages:

(a) Increased Productivity--more alcohol produced per batch

(b) Reduced Energy Requirement--distillation of the alcohol requiresless energy because there is more alcohol and less water in the beer.

(c) Increased capacity--the reflux of the column can be increased.

The increased efficiency of fermentation, using B.D.C. yeast,significant reduction in the time to reach equilibrium followingmolasses desalting is shown in the table below:

    ______________________________________                                                        Fermentation Max. EtOh                                               Rate of  Efficiency   Concentration                                           Alcohol  (hm EtOh/L/Hr)                                                                             After 16 h                                              Production                                                                             %            (% V/V)                                          ______________________________________                                        Non-clarified                                                                          2.0        78           4.5                                          molasses                                                                      Clarified                                                                              2.4        80           5                                            molasses                                                                      (same                                                                         installation)                                                                 Clarified                                                                              2.4        80           --                                           molasses                                                                      Clarified and                                                                          4.1        88           8.2                                          desalted                                                                      molasses                                                                      ______________________________________                                    

Another application of the present invention is the immobilization ofmineral crystals with total selectivity against NaCl. In other words tohave a membrane which is impermeable to NaCl, but which substantiallymaintains its water permeability; as indicated above, it is a feature ofbarriers created by the process of the present invention that they donot lose more than about 10% of their original permeability.

Such membranes could find potential use in, for example, concentrationof fruit juices (e.g. osmotic pressure about 5 atmospheres) in a runagainst sea water (osmotic pressure about 35 atmospheres), at absoluteminimal energy cost.

The invention further envisages the immobilization of crystals indiffusion barriers in dry form for use as gas permeation barriers. Theinventive process of the present invention enables the formation ofdiffusion barriers with a sufficiently well distributed latticestructure to provide selectivity between gases. Possible applicationsfor such barriers could be the separation of oxygen from the air, theextraction of helium, the purification of gases such as carbonic gas,etc.

Although the invention has been described above with reference tospecific examples and preferred embodiments, it will be appreciated thatthe invention is not limited thereby, and that variations are possiblewithout departing from the spirit or scope of the invention as broadlydescribed.

I claim:
 1. A diffusion barrier for selective counter diffusion,comprising a microporous matrix including a plurality of porous hollowfibre membranes and having one or more inorganic salts in the pores andon the inside surfaces of the cores of the hollow fibre membranes andforming an immobilized inorganic molecular grid or screen having asubstantially uniform lattice structure defining pores or channelsthrough the fiber walls having a diameter in the range of about 10 toabout 50 Angstroms, said diffusion barrier having substantially uniformpermeability.
 2. The diffusion barrier of claim 1, wherein theimmobilized inorganic molecular grid comprises a co-precipitate of twoseparate inorganic materials.
 3. The diffusion barrier of claim 2,wherein the immobilized inorganic molecular grid comprises co-crystalsof barium sulphate and an aluminium compound.
 4. The diffusion barrierof claim 3, including a protective coating on the immobilized inorganicmolecular grid.
 5. The diffusion barrier of claim 1, wherein theimmobilized inorganic molecular grid comprises a composite layer ofdifferent organic materials deposited sequentially onto and within saidpermeable matrix.
 6. The diffusion barrier of claim 5, wherein theimmobilized inorganic molecular grid comprises barium sulphate.
 7. Aprocess for the fractionation of a primary liquid containing a pluralityof dissolved molecular species of different molecular weights,comprising:(i) subjecting the primary liquid to a selective counterdiffusion treatment with a diffusion barrier which separates the primaryliquid from a second liquid, said diffusion barrier comprises amicroporous matrix having one or more insoluble inorganic salts in thepores of the matrix and forming an immobilized inorganic molecular gridor screen having a substantially uniform lattice structure definingpores or channels therethrough in the range of from about 10 Angstromsto about 50 Angstroms in diameter and substantially uniform matrixpermeability, said primary liquid and said secondary liquid beingselected such that an osmotic gradient exists between said primary andsecond liquids and an osmotic current flows through said barrier fromsaid second liquid towards said primary liquid and including the step ofcontrolling at least one of the shear rate of the primary liquid alongone side of said diffusion barrier and the osmotic gradient across saiddiffusion barrier such that small molecular species are selectivelytransferred from the primary liquid through said diffusion barrieragainst the osmotic current with a selectivity greater than the ratio ofthe relative diffusion coefficients of the separated species; and (ii)subsequently subjecting said primary liquid to ultrafiltration with asemipermeable membrane,permitting the passage therethrough of thesolvent component of said primary liquid together with molecules below apredetermined size contained therein, to form a permeate of treatedproduct, and preventing the passage of molecules above saidpredetermined size to form a concentrate or retentate of the remainingcomponents of the primary liquid.
 8. A process for the separation ofsmall molecular species from a sugar-containing solution, whichcomprises subjecting said solution to a selective counter diffusiontreatment with a diffusion barrier which separates the solution from asolvent, said diffusion barrier comprises a microporous matrix havingone or more insoluble inorganic salts in the pores of the matrix andforming an immobilized inorganic molecular grid or screen having asubstantially uniform lattice structure defining pores or channelstherethrough in the range of from about 10 Angstroms to about 50Angstroms in diameter and substantially uniform matrix permeability,said solvent and said solution being such that an osmotic gradientexists between the solution and the solvent, and an osmotic currentflows through said barrier from said solvent towards said solution andincluding the step of controlling at least one of the shear rate of thesolution along one side of the diffusion barrier and the osmoticgradient across the diffusion barrier such that small molecular species,including salts, are selectively transferred from the solution to thesolvent against the osmotic current with a selectivity greater than theratio of the relative diffusion coefficient of the separated species. 9.The process according to claim 8, wherein the diffusion barriercomprises a hollow fibre tube unit, wherein the solution is circulatedthrough the inner channels or lumens of a bundle of hollow fibres andwherein the solvent is circulated in the space surrounding the bundle ofhollow fibres, whereby the small molecular species can diffuse throughthe uniform molecular grid matrix structure in the walls of the hollowfibres against the osmotic current and into the solvent.
 10. A processaccording to claim 9, wherein the solvent circulating in the spacesurrounding the bundle of hollow fibres is further treated in a cascadesystem to separate the molecular species contained therein.
 11. Theprocess according to claim 10, wherein the solvent circulating in thespace surrounding the bundle of hollow fibres is circulated to the spacesurrounding the bundle of hollow fibres of a second counter diffusionhollow fibre tube unit wherein an osmotic gradient exists between thesolvent in said space and a further solvent circulating through theinterior channels or lumens of the hollow fibres of said second hollowfibre tube unit.
 12. The process according to claim 11, wherein thetreated solvent from the space surrounding the bundle of hollow fibresof said second hollow fibre tube unit is circulated back to the spacesurrounding the bundle of hollow fibres of the first hollow fibre tubeunit.
 13. The process according to claim 12, wherein a part of thetreated solvent from the space surrounding the bundle of hollow fibresof said second hollow fibre tube unit is circulated back to the streamof sugar-containing solution to be treated by the first hollow fibretube unit.
 14. The process according to claim 13, wherein pure solventis added to the circulating solvent stream to replace the volume ofsolvent which has been circulated back to the stream of sugar-containingsolution to be treated.
 15. The process according to claim 9, whereinthe solvent circulating in the space surrounding the bundle of hollowfibres is subjected to ion exchange or other treatment capable of fixingby adsorption the species to be removed from the solvent.
 16. Theprocess of claim 15 wherein the sugar-containing solution is selectedfrom the group consisting of, molasses and dunder from the fermentationof molasses.
 17. A process for the fractionation of a sugar-containingsolution which comprises:(i) subjecting said solution to a selectivecounter diffusion treatment with a diffusion barrier which separates thesolution from a solvent, said diffusion barrier comprising a microporousmatrix having one or more insoluble inorganic salts in the pores of thematrix and forming an immobilized inorganic molecular grid or screenhaving a substantially uniform lattice structure defining pores orchannels therethrough in the range of from about 10 Angstroms to about50 Angstroms in diameter and substantially uniform matrix permeability,said solution and said solvent being such that an osmotic gradient exitsbetween the solution and the solvent, and an osmotic current flowsthrough said barrier from said solvent towards said solution, andincluding the step of controlling at least one of the shear rate of thesolution along one side of the diffusion barrier and the osmoticgradient across said barrier such that small molecular species,including salts, are selectively transferred from the solution to thesolvent against the osmotic current with a selectivity greater than theratio of the relative diffusion coefficients of the separated species;and (ii) subsequently subjecting said solution to ultrafiltration with asemipermeable membrane,permitting the passage therethrough of water orother solvent component of said solution, together with sugar moleculescontained therein, to form a permeate of treated product containing saidsugar molecules, and preventing the passage of macro-molecular species,such as protein, contained in said solution to form a concentrate orretentate thereof.
 18. The process according to claim 17, wherein saidsugar-containing solution is molasses.
 19. The process according toclaim 17, wherein said sugar-containing solution is molassesdistillation residues or dunder.
 20. Apparatus for the fractionation ofa primary liquid containing a plurality of dissolved molecular speciesof different molecular weights, comprising:(i) a diffusion barrier forselective counter diffusion comprising a microporous matrix having oneor more insoluble inorganic salts in the pores of the matrix and on thesurfaces at one side of the matrix and forming an immobilized inorganicmolecular grid or screen having a substantially uniform latticestructure defining pores or channels therethrough in the range of fromabout 10 Angstroms to about 50 Angstroms in diameter, thus providing adiffusion barrier means having a substantially uniform matrixpermeability to enhance selectivity which is used to separate saidprimary liquid from a second liquid in a fractionation process wherebysmall molecular species may diffuse through the diffusion barrieragainst an osmotic current existing between said primary liquid and saidsecond liquid, from said primary liquid to said second liquid under theinfluence of an osmotic gradient between the primary and second liquidsto provide a selectivity greater than the ratio of the relativediffusion coefficients of the separated species; and (ii)ultrafiltration means whereby said primary liquid may subsequently beselectively split into a permeate of a product containing solvent andmolecules below a predetermined size, and a concentrate or retentatecomprising molecules above said predetermined size.
 21. The apparatusaccording to claim 20, wherein said diffusion barrier means comprisesone or more hollow fibre tube units, each comprising a tubular housing;a bundle of porous hollow fibres arranged within the housing in theaxial direction thereof; a countercurrent liquid chamber formed betweenthe outer surfaces of the hollow fibre bundle and the inner surface ofthe housing; first inlet and outlet ports for passing the second liquidinto and out of said liquid chamber; partition walls supporting thehollow fibre bundle, separating the open ends of the hollow fibres fromthe liquid chamber and defining the length of the liquid chamber; andsecond inlet and outlet ports for the primary liquid phase, said secondinlet and outlet ports communicating with the interior space or lumen ofeach of the hollow fibres, and wherein a plurality of pores or channelsof predetermined molecular dimensions and permeability communicatebetween the interior space or lumen of each hollow fibre and said liquidchamber.
 22. The apparatus according to claim 21, wherein the firstinlet and outlet ports of each hollow fibre tube unit are connected inseries to the first inlet and outlet ports of a second hollow fibre tubeunit for circulation of said second liquid therebetween.
 23. Theapparatus according to claim 22, wherein means are provided fordirecting at least part of the treated second liquid back to the streamof primary liquid to be treated by said apparatus, and wherein furthermeans are provided to add pure second liquid to replace the treatedsecond liquid which has been removed.
 24. The apparatus according toclaim 23, wherein ion exchange means or other means capable of fixing byadsorption is provided to remove compounds from the second liquid whichhave diffused therein from the primary liquid.
 25. A process for theseparation by counter diffusion of small molecular species from aprimary fluid containing a plurality of molecular species of differentmolecular weights, comprising:subjecting said primary fluid to treatmentwith a diffusion barrier for selective counter diffusion which separatesthe primary fluid from a second fluid, the primary fluid and secondfluid being such that an osmotic gradient exists between the primaryfluid and the second fluid and causes osmotic current to flow from saidsecond fluid through said barrier to said primary fluid, and includingthe step of adjusting one of the shear rate of the primary fluid alongone side of the barrier and the osmotic gradient across the barrierwhereby small molecular species are selectively transferred from theprimary fluid to the second fluid against the osmotic current with aselectivity greater than the ratio of the relative diffusion coefficientof the separated species, and said diffusion barrier comprising amicroporous matrix having one or more insoluble inorganic salts in thepores of the matrix and onto the surfaces at the said one side thereofto form an immobilized inorganic molecular grid or screen having asubstantially uniform lattice structure and substantially uniform matrixpermeability.
 26. A process according to claim 25, wherein:saiddiffusion barrier comprises a hollow fibre tube unit, and wherein theprimary fluid is circulated through the inner channels or lumens of abundle of hollow fibres, whereby the small molecular species can diffusethrough the uniform molecular grid matrix structure in the walls of thehollow fibres and be separated from the primary fluid.
 27. Apparatus forthe fractionation of a primary liquid containing a plurality ofdissolved molecular species of different molecular weights, comprising:adiffusion barrier for selective counter diffusion comprising amicroporous matrix having one or more insoluble inorganic salts in thepores of the matrix and onto the surfaces at one side thereof to form animmobilized inorganic molecular grid or screen having a substantiallyuniform, lattice structure defining pores or channels in the range offrom about 10 Angstroms to about 50 Angstroms in diameter, thusproviding a diffusion barrier means having a substantially uniformmatrix permeability to enhance selectivity, and which is used in afractionation process to separate said primary liquid from a secondliquid whereby small molecular species may diffuse through the diffusionbarrier, against an osmotic current existing between said primary andsecond liquids, from said primary liquid to said second liquid under theinfluence of an osmotic gradient between the primary and second liquid,to provide a selectivity greater than the ratio of the relativediffusion coefficients of the separated species.
 28. Apparatus accordingto claim 27, wherein:said diffusion barrier comprises one or more hollowfibre tube units, each comprising a tubular housing; a bundle of poroushollow fibres arranged within the housing in the axial directionthereof; a counter current liquid chamber formed between the outersurfaces of the hollow fibre bundle and the inner surface of thehousing; first inlet and outlet ports for passing the second liquid intoand out of the said liquid chamber; partition walls supporting thehollow fibre bundle, separating the open ends of the hollow fibres fromthe liquid chamber and defining the length of the liquid chamber; andsecond inlet and outlet ports for the primary liquid phase, said secondinlet and outlet ports communicating with the interior space or lumen ofeach of the hollow fibres, and wherein a plurality of pores or channelsof predetermined molecular dimensions and permeability communicatebetween the interior space or lumen of each hollow fibre and said liquidchamber.
 29. Apparatus according to claim 28, wherein the first inletand outlet ports of each hollow fibre tube unit are connected in seriesto the first inlet and outlet port of a second hollow fibre tube unitfor circulation of said second liquid therebetween.
 30. Apparatusaccording to claim 29, wherein means are provided for directing at leastpart of the treated second liquid back to the stream of primary liquidto be treated by said apparatus, and wherein further means are providedto add pure second liquid to replace the treated second liquid which hasbeen removed.
 31. Apparatus according to claim 28, wherein:ion exchangemeans or other means capable of fixing by adsorption is provided toremove compounds from the second liquid which have diffused therein fromthe primary liquid.
 32. A process for the separation by counterdiffusion of small molecular species from a primary liquid containing aplurality of dissolved molecular species of different molecular weights,said process comprising the steps of:providing a diffusion barriercomprising a microporous matrix having one or more insoluble inorganicsalts in the pores of the matrix and forming a substantially uniformstructure defining pores or channels therethrough having a diameter inthe range of about 10 to about 50 Angstroms; providing said primaryliquid on one side of said diffusion barrier; providing on the otherside of said diffusion barrier a solvent having a lower osmotic pressurethan the primary liquid, such that an osmotic gradient exists betweensaid solvent and said primary liquid and causes osmotic current to flowfrom said other side of said diffusion barrier through said diffusionbarrier to said one side thereof; causing said primary liquid to flowalong said one side of said diffusion barrier in a direction generallyparallel to said one side thereof; and, adjusting at least one of theshear rate of said primary liquid along said one side of said diffusionbarrier and said osmotic pressure gradient across said diffusion barriersuch that said molecular species are selectively transferred from saidprimary liquid to said solvent through said barrier against said osmoticgradient and current with a selectivity greater than the ratio of therelative diffusion coefficients of the separated species.
 33. Theprocess according to claim 32, wherein the diffusion barrier comprises ahollow fibre tube unit, wherein the primary liquid is circulated throughthe inner channels or lumens of a bundle of hollow fibres and whereinthe solvent is circulated in the space surrounding the bundle of hollowfibres, whereby the one other of said small molecular species candiffuse through the uniform molecular grid matrix structure in the wallsof the hollow fibres against the osmotic current and into the solvent.34. The process according to claim 33, wherein the solvent circulatingin the space surrounding the bundle of fibres is further treated in acascade system to separate the molecular species contained therein. 35.The process according to claim 34, wherein the solvent circulating inthe space surrounding the bundle of hollow fibres is circulated to thespace surrounding the bundle of hollow fibres of a second hollow fibretube unit wherein an osmotic gradient exists between the solvent in saidspace and a further solvent circulating through the interior channels orlumens of the hollow fibres of said second hollow fibre tube unitwhereby small molecular species are selectively transferred from thesolvent in said space to the further solvent circulating through theinterior channels or lumens of the hollow fibres of said second hollowfibre tube unit.
 36. The process according to claim 35, wherein thetreated solvent from the space surrounding the bundle of hollow fibresof said second hollow fibre tube unit is circulated back to the spacesurrounding the bundle of hollow fibres of the first hollow fibre tubeunit.
 37. The process according to claim 36, wherein a part of thetreated solvent from the space surrounding the bundle of hollow fibresof said second hollow fibre tube unit is circulated back to the streamof primary liquid to be treated by the first hollow fibre tube unit. 38.The process according to claim 37, wherein pure solvent is added to thecirculating solvent stream to replace the volume of solvent which hasbeen circulated back to the stream of primary liquid to be treated. 39.A diffusion barrier for selective counter diffusion, comprising amicroporous matrix including a plurality of porous hollow fibremembranes each having a wall thickness of not more than about 15 micronsand having one or more inorganic salts in the pores and on the insidesurfaces of the cores of the hollow fibre membranes to form animmobilized grid or screen having a substantially uniform latticestructure defining pores or channels through the walls of the fibremembranes having a diameter in the range of from about 10 Angstroms toabout 50 Angstroms and providing said diffusion barrier withsubstantially uniform permeability.
 40. A diffusion barrier forselective counter diffusion, comprising a microporous matrix having oneor more inorganic salts deposited in the pores and on the surfaces atone side of the matrix to form an immobilized grid or screen having asubstantially uniform lattice structure defining pores or channelsthrough the matrix having a diameter in the range of about 10 to about50 Angstroms, said diffusion barrier having substantially uniformpermeability.